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STATE  OF  ILLINOIS 
DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

A.   M.  SHELTON,   Director 

DIVISION  OF  THE 

STATE  GEOLOGICAL  SURVEY 

M.  M.  LEIGHTON,  Chief 

REPORT  OF  INVESTIGATIONS -NO.  9 

THE  GLENWOOD  BEDS  AS  A  HORIZON  MARKER 

AT  THE  BASE  OF  THE  PLATTEVILLE 

FORMATION 

BY 
ARTHUR  BEVAN 


PRINTED  BY  AUTHORITY  OF  THE  STATE  OF  ILLINOIS 


URBANA,  ILLINOIS 
192  6 


STATE  OF  ILLINOIS 
DEPARTMENT  OF  REGISTRATION  AND  EDUCATION 

A.  M.  SHELTON.  Director 

DIVISION  OF  THE 
STATE  GEOLOGICAL  SURVEY 

M.  M.  LEIGHTON.  Chief 


Committee  of  the  Board  of  Natural  Resources 
and  Conservation 

A.  M.  Shklton,  Chairman 

Director   of   Registration    and    Education 

Kendric  C.  Babcock 

Representing   the   President   of   the   Uni- 
versity of  Illinois 

Edson  S.  Bastix 
Geologist 


Schnepp&  Barnes,  Printers 

Springfield,  Ii.i.. 

1926 

1G416— 2M 


THE  GLENWOOD  BEDS  AS  A  HORIZON  MARKER  AT  THE 
BASE  OF  THE  PLATTEVILLE  FORMATION 

By  Arthur  Bevan 
OUTLINE 

PAfiE 

Introduction 5 

Purpose  of  the  report 5 

General    statement    5 

The   Glenwood    beds 6 

General  character  and   stratigraphic  relations 6 

Distribution  and  character  in  Illinois 7 

Character  in   the  Oregon   basin 8 

Stratigraphic   relations   in    Illinois .  11 

Geologic    history    12 

ILLUSTRATIONS 

FIGURE 

1.  Map  of  northern  Illinois  showing  the  approximate  line  of  outcrop  of  the 

Glenwood  beds  beneath   the  glacial  drift 4 

2.  Microphotographs  of  Glenwood  sandstone  in  the  vicinity  of  Oregon 9 

3.  Unconformity    between    the    St.    Peter    sandstone    and    Platteville-Galena 

dolomite    at    Deer    Park 12 


TABLE 

1.     Geologic  time  divisions  of  the  Paleozoic  era  and  epochs  of  the  Ordovician 

period     5 


GLKNWOOI)   BEDS   AS   A    HORIZON    MARKER 


Area  underlain  by  Platteville 
and  (or)  later  formations 

Area  underlain  by  St.  Peter  and 
Prairie  du  Chien  formations 

Approximate  line  of  outcrop  of  the 
Glenwood  beds  beneath  the  glacial 
drift 

Fig.  1.     Generalized    geologic   map   of   northern    Illinois, 
geologic   map   of   Illinois.) 


(Based    on    the 


INTRODUCTION 

Purpose  of  the  Report 

A  geologic  study  of  the  Oregon  quadrangle  in  the  northern  part  of  the 
State  and  a  review  of  the  pertinent  literature  of  northern  Illinois  and  ad- 
joining states  have  demonstrated  the  possibility  of  establishing  the  Glen- 
wood  beds  as  a  horizon  marker  between  the  St.  Peter  sandstone  and  the 
Platteville  limestone.  The  purpose  of  this  report  is  to  present  the  important 
characteristics  of  the  Glenwood  beds  as  an  aid  in  their  identification  in  out- 
crops and  in  drill  cuttings.  The  formation  is  described  briefly ;  its  value  as 
a  stratigraphic  key  is  emphasized,  and  the  geologic  conditions  under  which 
it  was  formed  are  discussed. 

General  Statement 

The  St.  Peter  sandstone  and  the  overlying  Platteville  limestone,  both 
of  middle  Ordovician  age  (see  Table  1),  are  widely  exposed  in  the  upper 
Mississippi  Valley.  They  crop  out  in  several  areas  in  northern  Illinois, 
southern  Wisconsin,  northeastern  Iowa,  and  southeastern  Minnesota.  Their 
striking  features  early  attracted  the  attention  of  geologists  working  in  this 
general  region,  and  the  chief  characteristics  of  each  have  been  described  in 
many  publications.      In  brief,  the  St.   Peter  is  generally  a  white  to   gray 

Taulk  1.     Geologic  time  divisions  of  the  Paleozoic  era  and  epochs  of  the 

Ordovician  period 


Era 


Period 


Epoch 


Paleozoic 


f  Permian 

j  Pennsylvanian 

Mississippian 

Devonian 

Silurian 


Ordovician 


Cambrian 


f  Cincinnatian    (Maquoketa) 
Galena 

Platteville    (Trenton) 
Glenwood 
St.  Peter 

Prairie   du    Chien    (Lower    Mag- 
nesian) 


b  GLEN  WOOD    BEDS   AS    A    HORIZON     MABKEB 

sandstone  composed  chiefly  of  medium  to  large,  fairly  well-rounded,  frosti 
rather  clean,  quartz  sand  grains,  which  are  commonly  very  loosely  cement) 
Its  lithology  is  rather  uniform  throughout  much  of  this  region;  but  call 
reous  and  shaly  beds  have  been  reported  from  a  few  horizons  at  a  f< 
localities.  The  Platteville  (the  Trenton  of  early  reports)  is  typically 
well-bedded,  fossiliferous,  buff  to  bluish,  somewhat  magnesian  lim<  ;1 
The  two  formations  are  thus  in  sharp  lithologic  contrast. 

tup:  glenwood  beds 

General  Character  and  Stratigraphic  Relations 

In  the  area  west  and  north  of  Illinois,  the  St.  Peter  and  the  Plat 
ville  are  commonly  separated  by  a  greenish  to  bluish  shale  ranging  in  thii 
ness  from  a  few  inches  to  a  few  feet.  This  intervening  formation  i> 
persistent  laterally  and  of  such  constant  character  in  eastern  Iowa  that  C 
vin  gave  it  the  name  of  Glenwood  shale.1  In  the  type  locality  this  sh 
is  1.")  feet  thick.  The  lower  two-thirds  is  highly  sandy  with  some  thin  In 
of  almost  pure  sand  like  that  in  the  underlying  St.  Peter  sandstone,  where 
the  upper  part  contains  no  sand.  Elsewhere  in  Iowa,  the  Glenwood  appe! 
to  be  a  clayey  shale,  commonly  3  to  <i  feet  thick.  Xo  trace  of  an  unci 
formitv  is  reported.  In  Minnesota,  the  change  from  the  St.  Peter  to  i 
Platteville  is  in  some  places  abrupt,  but  in  others  sandy  shale  or  calcar<  i 
sandstone  intervenes.  Sardeson  described  a  section  of  transition  beds  fr< 
the  bluff  of  Mississippi  Valley  at  Minnehaha,  which  is  \)A  feet  thick  a 
consists  of  alternating  thin  beds  of  green  clay  and  shale  with  thin  beds 
sand.  These  beds  contain  Platteville  fossils.-  Green  shale  is  reported  fr< 
other  localities  in  Minnesota.  Similar  beds  occur  in  southwestern  Wiso 
sin.  where  the  shale  is  more  or  less  sandy  and,  as  a  rule,  only  a  few  % 
thick.  According  to  Grant  and  Burchard,  the  relations  of  these  strata  -i 
gest  an  emergence  after  the  deposition  of  the  St.  Peter  sandstone. ::  In  i 
eastern  part  of  Wisconsin  south  of  Milwaukee,  a  "few  feet  of  more 
less  sandy  dolomite"  intervenes  between  the  typical  St.  Peter  and  a  30-f< 
stratum  of  "rather  coarse-grained  gray  calcareous  sandstone"  in  the  ba 
part  of  the  overlying  limestone.4 


'Calvin,    Samuel,    Geology   of    Winneshiek   County:    rowa    Geol.    Survey   Vol.   X 
pp.    74-75,    1906. 

!  Sardeson,    P.    W.,    T".    S.    Geol.    Survey    Geol.    Alias.     Minneapolis-St.     Paul    f< 
<  No.   201  ),   p.   6,   1916. 

'Grant,    l".    S..   and    Burchard,    P.    F.,    P.    S.    Geol.    Survej    Geol.    Atlas.    Lam 
Mineral   Point   folio   (Xo.   145),   p.   4.    1907. 

'Thwaites,    F.    T.,    Tin-    Paleozoic    rocks    found    in    deep    wells     in     Wisconsin    : 
northern    Illinois:    .lour.    Geol.,    vol.    XXXI.    p.    540. 


GLEN  WOOD   BEDS   AS  A   HORIZON    MARKER  7 

Distribution  and  Character  in  Illinois 

The  Glenwood  beds  crop  out  at  several  places  in  northern  Illinois — in 
the  vicinity  of  LaSalle,  in  the  Rock  River  drainage  basin  between  Dixon 
and  Oregon,  north  of  the  village  of  Leaf  River,  along  Elkhorn  Creek  west 
of  Haldane.  and  3  miles  northwest  of  Shirland  in  northern  Winnebago 
County  ( fig.  1 ).  The  exposure  of  these  beds  is  a  result  of  the  erosion  of  the 
crests  of  anticlines  such  as  the  LaSalle  anticline.  The  underlying  St.  Peter 
sandstone  is  also  exposed  in  each  one  of  these  areas.  In  the  LaSalle  quad- 
rangle Cady  reports  a  few  inches  of  shale  in  outcrops,  and  4  or  5  feet  of  shale 
in  wells  at  this  horizon.'  The  shale  is  absent  near  Troy  Grove,  north  of 
LaSalle,  there  being  a  transition  from  the  St.  Peter  to  the  Platteville 
through  a  foot  of  very  sandy  limestone.  The  Glenwood  in  the  Dixon 
quadrangle  is  a  green  shale.  'il/>  to  1  feet  thick,  whose  lower  beds  are  sandy 
in  places. ,;  The  formation  here  contains  considerable  potash.7  In  the 
Elkhorn  Creek  area  Hershey  notes  that  "there  is  a  six-foot  section  of  light 
green  shale  near  the  top  [of  the  St.  Peter],  which  is  persistent  throughout 
the  area  and  is  overlain  by  one  or  more  heavy-bedded,  partially  lithified 
strata  of  white  or  light  gray  sandstone,  constituting  the  top  of  the  forma- 
tion."1' The  lower  part  of  the  Platteville  is  "very  sandy  and  sometimes 
ihaly".  In  the  vicinity  of  Shirland  the  Glenwood  consists  of  green  sandy 
shale  with  sandy  limestone  at  the  top.9  An  analysis  of  the  deepest  green 
material  shows  0.51  per  cent  of  iron. 

As  shown  by  well  logs  the  Glenwood  horizon  is  represented  by  a  few 
feet  of  shale  at  places  in  northeastern  Illinois,  although  in  most  of  these 
lags  the  Platteville  limestone  is  indicated  as  immediately  overlying  the  St. 
Peter  sandstone.1"  Thin  intervening  beds  are  probably  present,  however, 
in  some  of  these  sections,  but  have  not  been  reported.  In  the  State  Hospital 
Avell  at  Kankakee,  15  feet  of  pure  to  sandy  magnesian  limestone  with  some 
green  shale  is  reported  as  separating  the  formations."  At  Woodstock,  41 
feet  of  sandy  limestone  occurs  above  the  St.  Peter.1-  The  log  of  a  well 
at  Lake  Forest  shows  that  "20  feet  of  dolomite  lies  beneath  the  upper  35 
feet  of  the  St.  Peter  sandstone,  but  the  passage  from  the  latter  to  the  Platte- 

s  Cady,  G.  H..  The  geolog-y  and  mineral  resources  of  the  Hennepin  and  LaSalle 
quadrangles,    111.    State  Geol.   Survey   Bull.    :!7.    pp.    39-40,    1919. 

•;  Knappen,   R.   S.,   oral  communication. 

T  Parr.  S.  W„  and  Austin,  M.  M.,  Potash  shales  of  Illinois  Univ.  of  111.  Agr. 
jgxp.    Sta.    Hull.     2:!2,    p.    236,    1921. 

1  Hershey,  O.  H.,  The  Elkhorn  '.'reek  area  of  St.  Peter  sandstone  in  northwest- 
Mi    Illinois:    Am.   Geol.,   vol.    XIV,   p.   174,   1S94. 

0  Lamar,    J.    10. ,    oral   communication. 

10 Anderson,  C.  B.,  Artesian  waters  of  northeastern  Illinois:  111.  State  Geol,  Sur- 
Bull.    ::4,    1919. 

"  Op.  cit.,  p.  it:.. 

'-  Op.  cit..   p.  211. 


8  GLENWOOD    BEDS   AS   A    HORIZON    MAI1KKK 

ville  is  abrupt.'''     A  well  at   Malta,  in  DeKalb  County,  shows  a  serii 
transition  beds  which  is    |.">   feet  thick,  as  given  in  the   following  section1*] 

Section  of  Malta  well  shovnng  character  of  transition  beds  between  th<   St.  Petei 
and  Platteville-Galena  formations 

Description  of  strata  Thickness 

F(  I  ! 

Limestone   (presumably  Platteville-Galena )    210 

Limestone,  sandy  2 

Sandstone is 

Shale,  sandy    2 

Shale,  gray    23 

Sandstone   ( St.  Peter )    321 

In  some  other  sections,  the  beds  between  the  St.  Peter  and  the  PlattevilM 
resemble  those  to  the  north  and  west;  that  is.  they  consist  mainly  of  greenisl 
shale  with  some  sand  in  the  basal  portion,  and  thus  constitute  rather  typical 
Glemvood  shale. 

Character  in  the  Oregon  Basin 

The  upper  St.  Peter  and  the  basal  Platteville  crop  out  in  the  Oregon 
quadrangle  mainly  along  the  north  rim  of  the  Oregon  basin.  This  is  a 
conspicuous  depression,  resulting  from  erosion  of  the  crest  of  a  low  anti- 
cline, which  extends  east  and  west  through  Oregon  and  for  3  to  4  miles 
in  either  direction.  Similar  outcrops  exist  in  a  small  area  on  the  wesl  sidl 
of  Rock  River  north  of  Oregon  and  a  few  scattered  small  exposures  appeal 
north  of  the  village  of  Leaf  River. 

Instead  of  being  a  shale  as  in  most  other  areas,  with  the  exception  of 
some  places  in  northeastern  Illinois  and  southeastern  Wisconsin,  the  (ilen- 
wood  in  the  Oregon  quadrangle  is  almost  entirely  a  typical  sandstone.  Shale 
forms  the  greater  part  of  only  a  single  section  near  the  western  end  of  the 
Oregon  basin,  where  the  formation  is  less  than  4  feet  thick.  Elsewhere 
it  is  a  bluish-green  sandstone  composed  of  a  variable  mixture  of  rather  line 
angular  sand  and  large  sand  grains  of  the  St.  Peter  type,  with  the  laitcr 
commonly  very  much  in  the  minority  (fig.  'i  ) .  The  sandstone  varies  some- 
what both  horizontally  and  vertically.1"'  It  ranges  in  thickness  from  '.' 
to  12  feet. 

On  account  of  the  persistence  of  the  bluish-green  color  and  the  charac- 
teristic texture,  the  Glemvood  beds  are  an  excellent  horizon  marker.  Evefl 
on  slopes  so  mantled  with  glacial  drift  that  neither  the  upper  St.  Peter  nor 
the  lower  Platteville  is  exposed,  in  many  places  the  position  of  the  (ilen- 

"Op.  cit.,  p.  ist;. 
"Op.    cit.,    ]>.    134. 

"The   details    of    this    variation    will    be    presented    in    tin-    111.    State    (ieol.    Survey 
bulletin   on   the   Oregon    quadrangle,    in   course   of  preparation. 


GLENWOOD   BKDS   AS  A   HORIZON   MARKER 


B 


m 

W0hw 


Fig.  2.    Microphotographs    of    Glenwood    sandstone    in    the 
vicinity  of  Oregon.     (Magnified'  about  16  times.) 

A.  On  the  west  slope  of  Liberty  Hill  northwest  of 
Oregon,  NE.  %  sec.  4.  T.  23  N.,  R.  10  E.  Shows  fine  angular 
sand  grains  of  the  Glenwood  with  large  rounded  grains  of 
the  St.   Peter  type. 

B.  In  head  of  ravine  about  3%  miles  east  of  Oregon. 
SW.  i*  sec  5,  T.  23  N.,  R.  11  E.  Similar  to  (A)  but  contains 
fewer    grains    of    the    St.    Peter    type. 


10  OLENWOOD   BEDS   AS   A    UORIZON    MARKEB 

wood  can  be  determined  readily  by  the  fragments  brought  to  the  surface 
by  burrowing  animals.  The  uniform  characteristics  of  the  formation  should 
make  it  a  useful  datum  plane  in  well  sections,  especially  in  those  areas  where 
the  boundary  between  the  St.  Peter  sandstone  and  the  Platteville  limestone 
is  not  sharply  marked. 

In  places,  the  Glenwood  appears  closely  related  to  the  St.  I'eter.  and 
the  contact  can  be  determined  only  by  the  change  in  color  and  the  abrupt 
change  from  typical  St.  Peter  sand  to  fine  angular  sand.  Certain  sections, 
however,  exhibit  a  slight  unconformity  at  the  base  of  the  Glenwood.  Oi 
the  other  hand,  it  appears  to  be  conformable  with  the  overlying  Platteville. 
This  is  shown  by  the  parallelism  of  the  Glenwood  and  Platteville  strata 
without  traces  of  erosion  along  the  contact  and  also  by  the  transition  beds 
which  in  places  occupy  the  Glenwood  horizon.  Although  the  change  to  the 
Platteville  is  generally  abrupt,  the  basal  beds  of  limestone  are  locally  more 
or  less   sandy  and   even   become   calcareous   sandstone   in  a    few   places. 

A  few  miles  east  of  Oregon,  the  Glenwood  sandstone  is  replaced  later- 
ally by  a  series  of  transition  beds,  which  on  the  whole  are  quite  dissimilar 
to  other  portions  of  the  formation  in  this  area  and  unlike  most  of  the  re- 
ported sections,  except  a  few  sections  in  wells  in  the  northeastern  pari  cl 
the  State.  Without  giving  a  detailed  description  of  the  sections,  the  chief 
features  of  these  transitional  beds  may  he  summarized  briefly.  In  a  ravine 
in  the  NW.  V4  SW.  T4  sec.  6,  T.  23  N..  R.  11  E.,  about  2}  '.  miles  eaj 
of  Oregon,  the  St.  Peter  sandstone  is  overlain  by  the  following  sequence  m 
strata  : 

Section  of  strata  overlying  the  St.   Peter  sandstone 
in   a   ravine  2%    mites  east  of  Oregon 

Description  of  strata  Thickness 

Ft.  In. 

Glacial  drift 

Sandstone;  rusty,  friable  mixture  of  round  and  angular  sand. 
slightly   calcareous    1 

Shale,  argillaceous,  greenish  to   purplish :! 

Sandstone,  light  gray  to  rusty,  more  or  less  calcareous  to  argil- 
laceous, composed  of  fine  angular  sand 13 

Limestone,  gray  to  buff,  somewhat  magnesian.  slightly  sandy. 
Some  beds  vermicular,  and  with  included  angular  limestone 
fragments 20  0 

Sandstone,  greenish,  of  fine  rounded  sand 1 

St.  Peter  sandstone 

The  total  thickness  of  the  preserved  section  of  transitional  beds  is  almost  in 
feet,  but  the  position  of  outcrops  of  Platteville  in  the  vicinity  show  that 
its  original  thickness  was  slightly  greater.  The  lowest  sandstone  is  the 
only  member  of  the  section  that  resembles  the  typical  Glenwood  in  coloa 


GLENWOOD   BEDS   AS   A    HORIZON    MARKER  11 

but  it  lacks  the  characteristic  texture,  whereas  the  other  sandstones  possess 
the  characteristic  texture  without  the  green  color. 

In  another  ravine  in  the  NW.  lA  SW.  YA  sec.  5,  T.  23  N.,  R.  11  E., 
a  complete  section  of  the  transition  beds  is  exposed.  The  succession  is  as 
follows  : 

Section  of  transition  beds  between  the  St.  Peter  sandstone  and  the  Plattcville  lime- 
stone in  a  ravine  in  sec.  5,  T.  23  N.,  R.  11  E. 

Description  of  strata  Thickness 

Ft.  In. 

Platteville  limestone 
Shale  and  clay,  bluish-green  to  yellow-brown,  with  some  limestone 

fragments    1  8 

Sandstone,  gray  to   white,   very   friable,   of  round   sand    scattered 

through  angular  sand    4  9 

Sandstone  and  limestone  in   alternating  beds.     The   sandstone   is 

greenish   and   very   fine-grained;    the   limestone   is  gray    to   buff 

and  magnesian   8  3 

Sandstone,  greenish    5  6 

Limestone,    buff,    dense 4  0 

Concealed     2  0 

St.  Peter  sandstone 

The  section  is  about  26  feet  thick,  and  it  rather  closely  resembles  the  typical 
Glenwood  sandstone  except  for  the  limestone  beds  in  its  lower  part. 

The  most  eastern  exposure  known  in  this  region  is  one  just  beyond  the 
southeastern  corner  of  the  Oregon  quadrangle  where  transition  beds  of 
thin  limestone,  sandstone,  and  bluish-green  shale  totaling  4^4  to  5J^  feet  in 
thickness  are  present  between   the   St.   Peter  and  the   Platteville."'' 

Stratigraphic  Relations  in  Illinois 

No  break  is  reported  at  the  top  of  the  St.  Peter  sandstone  in  Iowa 
and  in  parts  of  Minnesota,  Wisconsin,  and  northern  Illinois.  In  fact  the 
local  occurrence  of  beds  of  typical  St.  Peter  sand  in  the  upper  portion  of 
the  Glenwood,  as  reported  along  Elkhorn  Creek  in  northern  Illinois,  sug- 
gests that  in  some  areas  deposition  was  continuous  from  St.  Peter  time  into 
Glenwood  time.  Similar  evidence  is  afforded  in  places  by  the  alternation 
of  beds  of  typical  St.  Peter  sandstone  and  of  limestone  resembling  the 
Platteville,  as  well  as  by  the  transition  beds  described  in  this  paper  and  in 
other  papers  to  which  reference  has  been  made.  On  the  other  hand,  an 
erosional  unconformity  is  present  above  the  St.  Peter  in  southwestern  Wis- 
consin and  in  northern  Illinois,  as  for  instance,  near  Oregon.  At  Deer 
Park,  south  of  LaSalle,  the  contact  of  the  St.  Peter  and  the  Platteville  is 

16  Bretz,  J.  H.,  Geology  and  mineral   resources  of   the   KitiKs  quadrangle:    111.    State 
C.ol.    Survey    Bull.    4:!,    p.    224,    1923. 


12 


GLENWOOD    BEDS    AS    A    HORIZON    MARKER 


an  erosional  unconformity  with  a  relief  of  about  2  feet  in  a  short  distance 
(fig.  .'!).  Logs  of  wells  in  the  northeastern  part  of  the  State  contain  evi- 
dence of  a  break  at  this  horizon. 

Similar  conditions  exist  in  southwestern  Illinois  where  the  Glenwood 
and  the  overlying  Plattin  (or  Platteville)  limestone  rest  on  the  eroded  sur- 
face of  the  Joachim  limestone  in  Calhoun  County.17  The  absence  of  the 
latter  formation  in  the  northern  part  of  the  State,  particularly  in  view  of 
its  considerable  thickness  in  the  Ozark  region,  is  further  evidence  of  the 
extent  and  nature  of  the  unconformity. 

Although  the  lithologic  change  from  the  Glenwood  to  the  Platteville  is 
commonly  abrupt,  it  should  be  noted  that  no  trace  of  an  unconformity  be- 


Fig.  3.     Unconformity  between  St.  Peter  sandstone  and  Platteville-Galena  dolomite 
at  Deer  Park.    (Reproduced  by  permission  of  the  University  of  Chicago  Press.) 

tween  these  formations  has  been  reported.  They  seem  entirely  conformable 
in  the  Oregon  quadrangle.  They  are  closely  associated  in  southwestern  Wis- 
consin,18 and  Platteville  fossils  have  been  reported  by  Sardeson  from  strata 
at  the  Glenwood  horizon  along  the  upper  Mississippi. 


Geologic  History 

The  characteristics  of  the  Glenwood  beds  and  their  relation  to  the  St. 
Peter  sandstone  and  the  Platteville  limestone  indicate  the  geologic  condi- 
tions in  northern  Illinois  and  the  upper  Mississippi  Valley  during  this  por- 
tion of  the  Ordovician  period.     It  has  been  established  that   an  extensive 


17  Weller,    Stuart,    oral    communication. 
"Grant,    U.    S.,   and    Burchard,    K.    F.,    I'. 
Mineral    Point   folio    (No.    145),  p.    I,   1907. 


S.   Geol.   Survey  Geol.   Atlas.    Lancaster* 


GLENWOOD   BEDS   AS    A    HORIZON    MARKER  13 

marine  embayment  covered  this  region  during  St.  Peter  time.19  The  con- 
ditions immediately  following  the  deposition  of  the  St.  Peter  sands  have  not 
been  determined  conclusively  for  this  entire  region,  as  the  evidence  given 
in  the  literature  is  meager  and  indecisive  for  some  areas  and  even  appears 
somewhat  contradictory   when   different   districts  are  compared. 

On  the  basis  of  the  features  described  in  this  paper,  the  changing  geo- 
graphic conditions  during  this  episode  in  the  Ordovician  history  of  northern 
Illinois  may  be  interpreted  briefly.  After  a  considerable  interval  of  marine 
submergence  during  St.  Peter  time,  the  sea  withdrew  from  the  region,  thus 
permitting  slight  erosion  of  the  surface  of  the  recently  deposited  sands. 
The  apparent  absence  of  deep  erosion  channels  in  the  St.  Peter  sandstone 
may  indicate  that  the  land  remained  close  to  sea  level,  or  that  the  next 
sea  to  invade  the  region  reworked  the  uppermost  beds  so  completely  as  to 
obliterate  most  of  the  relief.  Rivers  and  winds  probably  transported  some 
sand  over  this  land  area.  After  a  brief  interval  the  sea  again  invaded  the 
region  with  the  result  that  the  unconsolidated  or  loosely  cemented  St.  Peter 
was  reworked  by  waves  and  currents  and  some  of  it  was  incorporated  in 
the  Glenwood  sandstone.  Many  of  the  typical  rounded  and  frosted  sand 
grains  in  the  latter  may  have  been  derived,  however,  from  the  original 
source  of  the  St.  Peter  sand.  The  much  smaller  size  and  the  angularity 
of  most  of  the  Glenwood  sand  seem  to  indicate  a  new  source  of  detritus  dur- 
ing this  time.  Perhaps  streams  were  transporting  sediment  from  the  crystal- 
line rocks  of  northern  Wisconsin  or  adjoining  regions.  Much  finer  sedi- 
ment was  carried  into  the  sea  south  and  west  of  the  Oregon  basin.  For 
very  brief  intervals  typical  St.  Peter  conditions  of  deposition  recurred,  either 
because  currents  were  sweeping  over  the  top  of  the  St.  Peter  sandstone  or 
because  winds  and  streams  were  deriving  sand  from  exposed  St.  Peter  or 
other  sandstones  to  the  north.  From  time  to  time  during  the  Glenwood 
stage,  conditions  were  favorable  for  the  deposition  of  calcareous  sediments, 
either  organically  or  biochemically,  in  protected  bays  or  in  barrier-locked 
areas  in  the  open  sea.  Shifting  currents,  oscillating  shores,  or  a  fluctuating 
supply  of  sediment  caused  an  interbedding  of  sands  and  limy  muds  in  some 
places.  Finally,  the  sea  appears  to  have  rapidly  submerged  much  of  the 
Mississippi  Valley  area,  bringing  in  an  abundance  and  great  variety  of  char- 
acteristic lime-secreting  invertebrates,  so  that  the  Platteville  calcareous  sedi- 
ments accumulated  to  a  considerable  thickness. 

19  For  an   excellent   discussion   and   interpretation   of   the   St.   Peter   sandstone    see 
Dake,   C.    Ii..    The    problem    of    the    St.    Peter    sandstone:    Mo.    Bur.    Mines   and    Metall., 
tech.  ser.,  vol.  fi.  No.  1,  1921. 


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